Apparatus for producing coils from films of insulating material, conductively coated in a vacuum

ABSTRACT

A method and apparatus are disclosed for producing rolls or coils of film of insulating material which has been conductively coated, in a vacuum, by vaporizing conductive material in direct electron beam bombardment and condensing the vapor on the film surface. According to the invention, the film is subjected to a plasma treatment after coating and before the film is wound in a coil.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing rolls or coils fromfilms of insulating material coated under vacuum with electricallyconductive material. The film is guided at a specified angle of contactover a roller rotating with the film and coating the film during itscontact with the roller by vaporizing the conductive material by directelectron beam bombardment and condensing the vapor on the film surface.After this procedure the coated film is coiled up on a take-up drum.

The method referred to above is known from the German published patentapplication No. 3,420,245. The film of insulating material is made froma synthetic thermoplastic material; the electrically conductive materialis a metal alloy of a magnetic material, such as a cobalt-nickel alloy.Magnetic tapes for audio and video recordings, for example, are producedby this known method.

This method of producing coated films is suitable not only for use withsuch films of insulating material in conjunction with electricallyconductive magnetic materials, but also for the production of so-calledcapacitor films, which comprise a substrate of thermoplastic film and analuminum layer. Neither the film material nor the electricallyconductive coating sets limits to the range of applications of theinventive method.

With such a method, the handling of the film on its path from thecoating source, that is, the electron beam vaporizer--to the take-updrum proves to be especially problematical. First of all, the film mustbe pulled from the roller, which frequently is also referred to as acooling roller or a vapor-deposition roller, with the expenditure of notinconsiderable forces, because negative charge carriers, which bringabout electrostatic charges, are implanted in the film by thevapor-deposition process, so that the film adheres to the roller due toelectrostatic forces. Admittedly, this may be considered a positivefeature insofar as the electrostatic "suction" promotes the heatdissipation to the roller and thus the cooling of the film. On the otherhand, however, it leads to considerable coiling problems, because thetensile stress produced by the effect described leads to extremelyundesirable fold formation in the film. The tendency to form foldsincreases wih the following parameters:

increasing voltage of the electron beam,

increasing film width,

increasing number of deflector rolls,

decreasing film thickness, and

increasing temperature difference (quenching effect).

An attempt has already been made to produce a stretching effect in thetransverse direction of the film with curved deflector rolls (theso-called "banana rolls") and, by so doing, to counteract the formationof folds. This measure also has only a very limited effect.

The absence of the electrically conductive coating at the two side edgesof the film has a particularly disavantageous effect. These twocoating-free edge strips--as well as the uncoated center strip--arerequired for capacitor films; they are, however, also especiallydesirable so as not to coat the length of roller which necessarilyprojects beyond the film, since this would lead to an edge bead thatbecomes increasingly thicker. The uncoated edge strips have aparticularly disadvantageous effect during the build-up of the coil, andmoreover, as the coil diameter increases, the build-up of a regular edgebead with a partial doubling over of the film may be observed. It hashitherto not been possible to eliminate this effect, so that only a coilwith a limited diameter could be produced.

Electrons may be regarded as the cause of the formation of folds and ofthe so-called edge beads on the coil. These electrons are reflected fromthe surface of the material to be vaporized from the so-called bathmirror, and have energies up to the energy of the electron beam. Theyare implanted in the film and can charge this film up to a voltage of 30kV. This charge causes the film to "adhere" not only to thevapor-deposition roller, but also to the subsequent deflector rolls. Onthe finished coil, this charge also leads to an attraction of theadjacent coil layers in the region of the conductive coating. It may beassumed that the cause of this attraction is a polarization within thecoated film. This attraction prevents the gliding process of theindividual film layers over one another, which is necessary for afold-free coil. On the other hand, when uncoated edge strips arepresent, repulsion of the individual film layers takes place in theregion of these strips and finally leads to the above-described build-upof an edge bead in the coil.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a coilingmethod of the initially described type, with which fold formationlargely does not take place or at least is highly suppressed and withwhich the aforementioned edge bead does not develop, especially not onthe coil.

This objective is accomplished, in accordance with the invention, bysubjecting the coated film to a plasma treatment on its way to thetake-up drum.

The expression "coated film" implies that this plasma treatment iscarried out following the electron beam vaporizer in the runningdirection of the film.

The plasma treatment quite obviously produces a very extensive, if notcomplete decay of the electrostatic charge, so that (1) the film can bepulled without difficulty from the vapor-deposition roll as well as fromthe subsequent deflector rolls and that (2) the necessary but slightcross movements can also be carried out on the coil, as it is in theprocess of being built-up, as a result of which fold formation isprevented. In particular it has turned out that the undesirable edgebead is not built up on the coil with the inventive plasma treatment.

The plasma treatment may be carried out at any place in the path of thefilm before the film runs up on the coil. It is, moreover, possible toexpose the film on the coating side, on the rear side or on both sidesto the plasma treatment. It is even possible to treat the film with theplasma only in its two edge regions, either on the front or the rearside or on both sides.

It is, however, particularly advantageous if the plasma treatment iscarried out at the end of the angle of contact, especially if the plasmatreatment is carried out before the film is pulled from the roller. Inthis case, namely, the electrostatic "adhesion" to the vapor-depositionroll is nullified in a timely manner, so that an excessive tensilestress at the film is precluded.

Various devices come into consideration as a plasma source: for example,disk-shaped electrodes, which are flat or curved concentrically with thevapor-deposition roll and which are connected to a negative voltage ofbetween 500 and 5,000 volts. The voltage may be a direct-currentvoltage, an alternating-current voltage or a high frequency voltage. Thematerial, preferably used for the electrode, is one which is notatomized readily, such as aluminum or high-grade steel. It is, however,particularly advantageous to use a so-called magnetron cathode, in whicha closed tunnel of lines of magnetic flux is formed over a plate of ametal that is not readily atomized. In principle, such an arrangement iswell known from so-called sputtering cathodes. Such magnetrons aredescribed, for example, in the German published patent application No.2,243,798. They may likewise be supplied either with direct-currentvoltage, an alternating-current voltage or with a high frequencyvoltage. When a magnetron cathode is used, the voltage can be limited tovalues between -300 and -800 volts.

It is, moreover, advisable to ensure that the plamsa contacts thesurface of the film. Since in the case of a magnetron cathode the plasmais held in the vicinity of the cathode by the magnetic tunnel, themagnetron cathode must be brought closer to the film than a cathodewithout magnetic field support.

The specific wattage, based on the effective cathode surface, isadvisably selected to be between 0.1 and 5 watt/cm² and typically atabout 2.5 watt/cm². The residence time, during which the film passesthrough the plasma, is advisably selected to be between 0.2 and 10milliseconds.

The invention also relates to equipment for implementing the inventiveprocess. Such equipment has a feed drum, a roller disposed above theelectron beam vaporizer and looped by the film and a take-up roll, aswell as deflector rolls for establishing a path for the film.

To accomplish the same objective, at least one plasma source is disposedpursuant to the invention in the region of the path of the film from theelectron beam vaporizer to the take-up drum. This plasma source is sooriented towards the path of the film, that the plasma contacts thefilm.

For a full understanding of the present invention, reference should nowbe made to the following detailed description of the preferredembodiment of the invention, taken in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a cross-sectional, and partly schematic diagram ofthe apparatus, according to the present invention, for coating film ofinsulating material with electrically conductive material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows a vacuum chamber 1 in which a feed drum 2, a roller 3(vapor-deposition roll), through which a coolant is flowing, and atake-up drum 4 are disposed parallel to the axis. The path of a film 5is determined by the feed drum, the roller and the take-up drum, as wellas by deflector rolls 6, 7, 8 and 9, it being understood that, becauseof the change in the diameter of the coil, the path of the film 5between the feed drum 2 and the deflector roll 6, as well as between thedeflector roll 9 and the take-up drum 4 is variable. In practicalconstructions, further deflector rolls are usually present. For the sakeof simplicity, however, they have not been shown here. At least thetake-up roll 4 is driven.

The angle of contact of the roller 3 is the larger angle between the twodot-dash lines 10 and 11. In the region of this angle of contact, thereis first of all a glow-discharge device 12 for preparing or cleaning thefilm for the actual coating process. Such glow-discharge devices arestate of the art and are therefore not described herein in detail.

Below the roller 3, there is an electron-beam vaporizer 13, whichcomprises an electron-beam gun 14 and a liquid-cooled vaporizer crucible15. The vaporizer crucible has a rectangular recess, in which the fusedvaporization material 16 is contained during the operation of theequipment. The upwards pointing opening of the crucible also isrectangular and its longest axis of symmetry runs perpendicular to theplane of the drawing and parallel to the axes of rotation 2a, 3a and 4aof the drums and the roller. Since the present case involves the vapordeposition of a magnetic material on the film 5, a normal(perpendicular) line, passing through the center of the crucibleopening, is displaced transversely to the axis of rotation 3a, so thatthe desired angular vapor deposition takes place.

In the region of the roller 3, the vacuum chamber 1 is subdivided by adiscontinuous partition 17, so that two partial chambers 1a and 1b areformed, which are linked over two vacuum connection pieces to theassociated vacuum pumps. The partition serves the purpose that theoutgassing of the film on the feed drum is restricted to the partialchamber 1b and that the vapor-deposition process is restricted to thepartial chamber 1a.

Within the partial chamber 1b, three plasma sources 20, 21 and 22 areshown, which are constructed as so-called magnetrons and one of whichsuffices in the limiting case. As may be seen in the FIGURE, a groundingshield 20a is arranged to surround the plasma source 20. Such a shieldis also present on the other plasma sources 21 and 22 but, for the sakeof simplicity, has been omitted in the drawing. The plasma sources 20 to22 each consist of a closed box 23, the front side 23a of which isoriented towards the film 5 and comprises a material with poor atomizingproperties (aluminum, high-grade steel. The lines of flux of a closedmagnetic tunnel, which are shown by broken lines, penetrate through thisfront side. As already stated, the mode of action of such a magnetictunnel is described in the German published patent application No.2,243,708.

The plasma source 20 is disposed at the end of the angle of contactpreset on one side of the deflector roll 7. Moreover, the plasma source20 is oriented with its plasma-generating electrode surface, the frontside 23a, towards the axis 3a of the roller 3. Strictly speaking, thesurface normal, passing through the center of gravity of the front side23a, is oriented towards the axis 3a. As a result, the front side 23aruns tangentially to the immediately opposite surface section of roller3 or to the partial length of film 5 on this section. The adhesiveeffect of the film 5 on the roller 3 is substantially or even completelyeliminated by the plasma source 20 even before the film is lifted offthe roller. The plasma sources 21 and 22 may be additionally present,either individually or in pairs; they may also be present alternativelyor be omitted. In the limiting case, one of the plasma sources 20, 21 or22 suffices. It can be seen that the path of the film leads through themagnetic fields of the plasma sources. In any case, the spatialadjustment is made so that plasma, held within the magnetic tunnel,reaches the film. With the usual design of such magnetrons, this is thecase when the distance between the front side 23a and the film 5 isbetween 20 and 50 mm.

EXAMPLE 1: (Comparison Example)

In equipment like that shown in the FIGURE, a polyester film, 52 cm wideand 9-12 μm thick, was sputtered with a cobalt-nickel alloy in order toproduce video tapes. The running speed of the film and thus thecircumferential speed of the roller 3 was 100 m/min. The wattage of theelectron beam vaporizer 13 was adjusted so that the thickness of theCoNi alloy layer on the film was 100 nm. Since none of the plasmasources 20 to 22 was put into operation here, a coil with many folds andwith two strong edge beads, one at either end of the coil, resulted. Thecoiling process had to be terminated after the coil reached a diameterof 25 cm. The width of the uncoated edge at either side was about 1 cm.

EXAMPLE 2

The experiment of Example 1 was repeated, however with the differencethat the plasma source 20, the length of which was larger by somecentimeters than the width of the film 5, was supplied with a voltage of-300 volts. With this arrangement, the cathode current was 10 amps. Atthe same time, argon was supplied in such amounts to the partial chamber1b, that an argon partial pressure of 5×10⁻³ to 1×10⁻² mbar developed.The specific wattage of the magnetron, based on the surface area of thefront side 23a, was 2.5 W/cm². A coil, practically completely free offolds, was formed on the take-up drum 4 and, in particular, no edge beadwas formed on either side of the coil. It was possible to continue thebuild-up of the coil undisturbed up to a total diameter of 40 cm.

EXAMPLE 3

The experiment of Example 2 was repeated, however, with the differencethat an aluminum layer for producing a capacitor film was sputtered on apolyethylene film 52 cm wide and 6 μm thick. The running speed of thefilm here was 480 m/min and the wattage of the electron beam vaporizer13 was adjusted, so that a layer 350 nm thick resulted. The plasmasource 20 was operated with the same parameters as in Example 2.However, because the running speed was higher by a factor of 4.8, acorrespondingly shorter residence time of the film in the region of theplasma discharge of the plasma source 20 resulted. An edge region of 1cm on both sides of the film was also kept free here of the aluminumcoating. Here also, a largely fold-free coil was formed on the take-updrum 4 which, in particular, was also free of edge beads and it waspossible to build up the coil undisturbed up to a diameter of 40 cm.

EXAMPLE 4

The experiment of Example 3 was repeated. However, to compensate for thereduction in the residence time in the region of the plasma source 20due to the higher running speed of the film, additional plasma sources21 and 22, with a specific wattage 1/5 of that of the plasma source 20,were placed into operation. Strictly optically, there resulted an evensmoother build up of the coil on the take-up drum 4. However, theconditions in the edge region of the coil could not be improved further,since optimum results had already been achieved by the experimentdescribed above in Example 3.

There has thus been shown and described a novel method and apparatus forconductively coating films of insulating material which fulfill all theobjects and advantages sought therefor. Many changes, modifications,variations and other uses and applications of the subject inventionwill, however, become apparent to those skilled in the art afterconsidering this specification and the accompanying drawing whichdisclose the preferred embodiment thereof. All such changes,modifications, variations and other uses and applications which do notdepart from the spirit and scope of the invention are deemed to becovered by the invention which is limited only by the claims whichfollow.

We claim:
 1. In a method for producing and coiling films of insulatingmaterial coated under a vacuum with electrically conductive material byguiding the film at a specified angle of contact over a roller rotatingwith the film and coating the film during its contact with the roller byvaporizing the conductive material by direct electron beam bombardmentand condensing the vapor on the film surface and thereafter coiling upthe coated film on a take-up drum, the coated film being exposed to aplasma treatment along its path to the take-up drum, the improvementwherein said plasma treatment is carried out at the end of the angle ofcontact with the roller, before the film is pulled off the roller, andthe plasma treatment is carried out by means of a magnetron cathode, theplasma contacting the film.
 2. The method defined in claim 1, whereinthe plasma treatment is carried out only in the edge regions of thefilm.
 3. The method defined in claim 1, wherein the plasma treatment iscarried out at a specific wattage of 0.1 to 5 watt/cm² of the particularcathode surface which causes the formation of the plasma.
 4. The methoddefined in claim 3, wherein the residence time of the film in the plasmaduring its passage through the plasma is 0.2 to 10 ms.
 5. The methoddefined in claim 1, wherein the plasma treatment is carried out in anonreactive atmosphere at a pressure between 5×10⁻² and 10⁻³ mbar.